1. Field of the Invention
The present invention relates to a manufacturing method of semiconductors, in particular detail, to a method for forming gate insulators in MIS (MOS) semiconductor devices, and to a manufacturing method of semiconductor devices provided with insulator films such as gate insulators on a surface of a silicon substrate.
2. Description of the Related Art
Recently, as MIS (MOS) semiconductors have been patterned finer, extremely thin gate insulators such as approximately 4 nm or less are in demand. So far, for gate insulator material, silicon oxide films (SiO2 film) have been industrially used that can be obtained by directly oxidizing a silicon substrate by use of a high temperature furnace of approximately 850xc2x0 C. to 1000xc2x0 C. 
However, when the SiO2 layer is 4 xcexcm or less, a leakage current (gate leakage current) flowing the gate insulator increases to cause problems such as an increase of consumption power or an acceleration of deterioration of device property.
In addition, there is such a problem that during formation of a gate electrode, boron contained therein causes alloy spikes in the SiO2 film to reach the silicon substrate to result in deterioration of semiconductor device property. As one method for solving such a problem, nitride film (SiN film) is under consideration as the gate insulator material.
When the SiN films are deposited by use of CVD method, there occur many incomplete bonds (dangling bond) at the interface with the silicon substrate to result in deterioration of device property. Accordingly, in forming the SiN films, it is considered very promising to directly nitride a silicon substrate by use of plasma. The reason why to nitride directly is to obtain gate insulators of high quality that are less in interface states.
In addition, one reason for using the plasma is to form SiN films at low temperatures. In obtaining SiN films by heating to nitride, high temperatures of 1000xc2x0 C. or more are necessary. In the process of the heating, dopant is injected into the silicon substrate. The dopant diffuses differentially to cause deterioration of device property. Such methods are disclosed in Japanese Patent Laid-open Application (KOKAI) Nos. SHO 55-134937 and SHO 59-4059.
However, in the case of depositing SiN layers with the plasma, the following problems have been pointed out. That is, ions in the plasma are accelerated by a plasma sheath voltage to bombard the silicon substrate with high energy, thereby so-called plasma damage occurs at interfaces of the silicon substrate or on the silicon substrate to deteriorate the device property.
To this end, a microwave plasma device is disclosed that is provided with a planar-array antenna that is low in electron temperature and has a lot of slits causing less plasma damage.
(Ultra Clean Technology Vol.10 Supplement 1, p.32, 1998, published by Ultra Clean Society).
In this plasma device, the electron temperature is approximately 1 eV or less and the plasma sheath voltage also is several volts or less. Thus, compared with existing plasma of which plasma sheath voltage is approximately 50 V, the plasma damage can be largely reduced.
However, even when silicon nitride is formed with this plasma device, in the case of forming SiN films by use of direct nitriding method, there is the following problem. That is, in order to obtain interfaces of good quality of less dangling bond defects by dominantly distributing oxygen only at the interfaces of the silicon substrate, there is a difficulty in regulating film quality at the interfaces with the silicon substrate.
In addition, in employing this plasma device to nitride, nitrogen atoms must diffuse into the silicon substrate to proceed nitriding. That is a slow process to require a long time to give prescribed processing to an object being processed. Accordingly, the objects can not be processed much per unit period to cause difficulties in industrial application. In forming SiN films of a thickness of for instance 4 nm, even under the best adjusted plasma conditions of such as pressure and microwave power, it takes approximately 5 min or more to process. Accordingly, throughput is much lower than that required from a viewpoint of mass-production, for instance 1 min per one piece of the object.
The present invention is made to solve the aforementioned problems. That is, an object of the present invention is to provide a method and an apparatus for manufacturing semiconductors that can successfully regulate film quality at the interfaces between silicon substrates and SiN films.
The other object of the present invention is to provide a method and an apparatus for manufacturing semiconductors that can form a SiN film of high quality in a short time.
To the above ends, a manufacturing method of semiconductors of the present invention is characterized in implementing the invention in the following manner. That is, in an atmosphere of processing gas, microwaves are irradiated through a planar-array antenna having a plurality of slits on an object to be processed comprising silicon to generate plasma containing oxygen, or nitrogen, or oxygen and nitrogen. With the plasma, direct oxidation, nitriding, or oxynitriding is implemented on a surface of the object to form an insulator film of a thickness of 1 nm or less (in terms of silicon oxide film).
In the present manufacturing method, a thickness of insulator film is 1 nm or less. Accordingly, the nitriding of the silicon substrate is not due to diffusion but due mainly to a reaction process between nitrogen atoms or oxygen atoms or nitrogen and oxygen atoms generated by the plasma and the surface of silicon substrate. As a result of this, a nitriding rate of such short as approximately 30 sec can be obtained.
On the thin insulator film that is obtained by implementing the direct nitriding or oxidizing or oxy-nitriding, the rest of the insulator film is deposited by use of CVD method. In this case, since a deposition rate of 3 nm/min or more can be attained relatively easily, even an insulator film of a total film thickness of 4 nm can be formed in less than two min.
In addition, in the present manufacturing method, a process for forming, due to direct nitriding or oxidizing or oxy-nitriding, an insulator film of good quality at an interface with the silicon substrate and a process for forming thereon, due to CVD method, the rest of the insulator film can be independently implemented. Accordingly, compared with the case where all process is implemented by direct nitriding only or CVD method only to form an insulator film, the film quality at the interface with the silicon substrate can be improved in regulation to result in an insulator film of better quality.
In the present manufacturing method, for the processing gases, a gas containing for instance N2 or N2O or NO or NH3 can be cited. The processing gas can contain rare gas such as argon or the like.
Another manufacturing method of semiconductors of the present invention comprises a step of forming a first insulator film and a step of forming thereon a second insulator film. Here, the step of forming the first insulator is carried out in the following manner. That is, in an atmosphere of a processing gas, on an object to be processed comprising silicon, through a planar-array antenna having a plurality of slits, microwaves are irradiated to generate plasma containing oxygen, or nitrogen, or oxygen and nitrogen. With the plasma, direct oxidizing, nitriding, or oxy-nitriding is implemented to form the first insulator film.
In the aforementioned manufacturing method, the second insulator film can be an insulator film comprising for instance silicon nitride.
The process of forming the second insulator film may be implemented by use of CVD method, or by use of plasma irradiation.
Plasma containing for instance N2 or NH3 and monosilane or dichlorosilane or trichlorosilane is supplied to form the second insulator film.
According to the present method, in an atmosphere of a processing gas, on an object to be processed consisting mainly of silicon, microwaves are irradiated through a planar-array antenna having a plurality of slits, so-called RLSA (Radial Line Slot Antenna) antenna. Thereby, the plasma is directly supplied on the silicon substrate to form a SiN insulator film. Accordingly, film quality at the interface with SiN insulator film formed on a surface of the silicon substrate can be successfully regulated.
Furthermore, according to another manufacturing method of the present invention, on a first insulator film formed by use of so-called RLSA antenna, all the second insulator film can be formed by irradiation of the plasma of low damage. As a result of this, a SiN film of high quality can be formed. In particular, when the second insulator film is formed by use of the CVD method, the insulator film can be deposited in a short time to result in formation of a SiN film of high quality in a short time.
In addition, in a silicon semiconductor device, so far, as a gate insulator, silicon oxide film (SiO2 film) has been used. However, when a thickness of SiO2 film is made thinner than 60 angstroms that is a thickness being employed now, there is a lower limit at 40 angstroms. When tried to make thinner than this, a leakage current becomes larger to result in larger power consumption. This is impractical.
Therefore, a silicon nitride film (SiN film) that does not cause a large leakage current when thinning down to approximate 40 angstroms is being considered to use as a gate insulator.
For instance, Japanese Patent Laid-open Application (KOKAI) Nos. HEI 5-36899 and HEI 9-50996 disclose an example of stacking a silicon nitride film due to thermal nitriding and silicon nitride film due to vapor phase growth method. In an example disclosed in HEI 5-36899, polycrystalline silicon is patterned in a prescribed shape to form an electrode, followed by fast thermal nitriding at 850xc2x0 C. for 60 sec with an annealing furnace to form a silicon nitride film of a film thickness of approximate several nm on a surface of the electrode due to thermal nitriding. On the surface of this silicon nitride film, a silicon nitride film of approximately 4 nm is deposited due to low-pressure vapor phase growth method.
In Japanese Patent Laid-open Application (KOKAI) No. HEI 6-61470, an example employing a silicon oxynitride film is disclosed. In this example, silicon oxide film is annealed in an atmosphere of NH3 at 900 to 1000xc2x0 C. for approximately 10 min to 1 hour to form silicon oxynitride film.
Furthermore, in Japanese Patent Laid-open Application (KOKAI) No. HEI 10-178159, an example of a combination of three layers of silicon oxynitride film, silicon nitride film, and silicon oxynitride film is disclosed. In this example, silicon oxynitride film is formed in the following way. In a low pressure CVD device, from monosilane and nitrous oxide, high temperature silicon oxide film is formed under conditions of a pressure of approximately 50 Pa and a temperature of 700 to 850xc2x0 C. Then, at a temperature of 700 to 850xc2x0 C., nitrous oxide is introduced to transform the high temperature silicon oxide film to silicon oxynitride film. On the other hand, the silicon nitride film is formed in a low pressure CVD device from dichlorosilane and ammonia at a temperature of 700 to 850xc2x0 C. 
However, silicon nitride film due to thermal nitriding has a lot of dangling bonds (free species) and is poor in electrical properties. Silicon nitride film (silicon nitride film) due to low-pressure vapor phase growth method also is poor in electrical properties. In addition, silicon oxynitride film takes a long time to form.
The present inventors used high-density plasma to generate plasma from a mixture of argon gas and nitrogen gas and hydrogen gas to form therewith SiN film by nitriding a surface of a silicon substrate. Although SiN film of excellent electrical property can be obtained by this means, there is a disadvantage that SiN film can not be formed with a high rate.
The present invention is made under such circumstances. An object is to provide a method for manufacturing semiconductor devices provided with an insulator film of excellent electrical property and of higher deposition rate.
A method for producing semiconductor devices of the present invention comprises a step of forming a first silicon nitride film and a step of forming a second silicon nitride film of larger deposition rate than that of the first silicon nitride film. Here, the first silicon nitride film is formed by nitriding a surface of a silicon substrate with plasma. The plasma is obtained from a gas mixture containing rare gas and nitrogen and hydrogen or rare gas and ammonia but not silicon and containing 50% and more and 99% or less of rare gas. The second silicon nitride film is formed on the surface of the first silicon nitride film with plasma, the plasma being obtained from a gas mixture containing rare gas and nitrogen and silicon and containing 50% or more and 99% or less of rare gas. At this time, it is preferable to generate the plasma with high frequency power of 300 MHz or more and 2500 MHz or less.
In a manufacturing method of semiconductor devices of the present invention, insulator film formed at the interface of a silicon substrate is silicon oxide film. The silicon oxide film may be formed by oxidizing a surface of the silicon substrate with plasma or without plasma, the plasma being obtained from a gas mixture containing rare gas and oxygen but not silicon and containing 50% or more and 99% or less of rare gas. Further, an insulator film formed at the interface of the silicon substrate may be silicon oxynitride film.